Bolboschoenus maritimus belongs to a difficult, worldwide complex, in which the delimitation of specific and infraspecific taxa is still unclear. The new lectotype and epitype from the Baltic coast of Sweden selected by S. G. Smith and I. Kukkonen (1999) are used here as the basis for redefining B. maritimus subsp. maritimus, which was previously defined to include B. yagara (Ohwi) Y. C. Yang & M. Zhan (J. Browning et al. 1996; Z. Hroudová et al. 1998).

It seems likely that some populations of both Bolboschoenus maritimus subsp. maritimus and B. maritimus subsp. paludosus have been introduced into the flora from Eurasia.

Putative Bolboschoenus maritimus × B. robustus hybrids are common in zones of sympatry along the northern Atlantic Coast and in California, putative B. fluviatilis × B. maritimus hybrids occur in California, and many B. glaucus × B. maritimus hybrids occur in California and Idaho (J. Browning et al. 1995). They are intermediate between their putative parents in every character studied and are often fertile. Interspecific hybridization is probably responsible for much of the infraspecific variation in B. maritimus, especially along the northern Atlantic and California coasts. The orange anthers and few 3-fid styles on some specimens from southeastern Texas may be due to introgression from B. robustus. The type of Scirpus maritimus forma agonus Fernald is probably a B. maritimus × B. robustus hybrid.